I don't think there's any sure way to tell, and trying to do so in the past has massively disadvantaged people who have English as a second language & get accused of plagiarism via LLM (can't find source with a quick google, but there was an ESL student accused of plagiarism by an 'ai text detection' model).

I think we just have to embrace the fact that any text could be wholly/partially LLM generated. An LLM could be asked to introduce errors (or a human could edit the output, either to purposefully introduce errors, or naturally introduce errors as part of their process). We should focus on:

1. Developing our skills to verify people's claims (which has always been important)

2. Trying to understand the motivations/agenda behind the text (which I reckon is even more important still)

The above two steps insulate you from a wide range of propaganda/trickery, and are the sorts of things we should have been focusing on in schools for decades already.

Mentioned elsewhere, bad grammar could be a human telling an AI to be plausibly bad.